The present invention concerns a device for moving the back rest in weaving machines, in particular according to a periodical movement.
It is known that, in weaving machines, the back rest has to move to and fro in accordance with the harness motion, this in order to obtain that, in spite of the opening and closing of the shed, the warp tension remains practically the same. As known from U.S. Pat. No. 3,567,354, the required periodical movement to this end can be obtained by means of the connecting rods which are connected to the back rest and driven by the sley.
It is also known that said periodical movement of the back rest can be obtained by means of an eccentric, driven by the main shaft of the weaving machine. This known embodiment is usually also provided with adjusting means to set the eccentricity. The setting of the eccentricity may involve a phase shift between the movements made respectively by the harnesses and the back rest. It is obvious that the phase between said movements has to be reset.
In order to obtain that the warp tension remains practically the same in spite of the opening and closing of the shed, the movement of the back rest has to be more or less in phase with the opening and closing of the shed. It is known, however, that, depending on the cloth to be woven, the movement of the harnesses in relation to the movement of the sley or the main shaft of the weaving machine may be changed. In order to obtain, in case of such a change, that said movement of the back rest, which has been obtained by the movement of the main shaft, remains in phase with the opening and closing of the shed, also the phase between the main shaft and the movement of the back rest has to be set.
In the known embodiments, this is done by driving the eccentric by means of a gear wheel clutch, such that, through the release of these clutches and through the mutual turning of the gear wheels, the setting can be executed.
It is clear that this is a time-consuming method which does not allow for continuous setting.